As is known, now available are numerous metrology systems, which find use, amongst other things, in the aerospace sector. In particular, in the aerospace sector, metrology systems are known having the function of determining the attitude and/or position of a satellite. Even more in particular, metrology systems are known which enable, given a pair of satellites, determination of the mutual position and mutual attitude of the two satellites.
Determination of the attitudes and positions of satellites is of particular importance in the case of satellite systems the satellites of which are arranged in formation, i.e., in the cases where there is envisaged the determination of the attitude and position of each satellite as a function of the attitude and position of the other satellites.
In practice, given two satellites, determination of the mutual attitude and mutual position requires determination of six degrees of freedom. In fact, assuming a first reference system and a second reference system fixed with respect to a first satellite and a second satellite, respectively, and formed, each, by a triad of perpendicular axes, the mutual attitude and mutual position of the first and second reference systems, and hence of the first and second satellites, can be expressed in terms of three (linear) displacements and three rotations (angles). In particular, the mutual position of the first satellite with respect to the second satellite can be expressed by means of a set of three displacements measured, respectively, along the three axes of the second reference system. Likewise, the mutual attitude of the first satellite with respect to the second satellite can be expressed by means of a set of three angles, equal to corresponding rotations of the first reference system with respect to the second reference system.
This being said, in general optical metrology systems now available can be divided into so-called “coarse” systems and so-called “fine” systems, according to the accuracy and the field of application, the latter being given by the range of distances that can lie between the satellites without the levels of performance degrading significantly.
In greater detail, fine metrology systems enable determination of the mutual position of two satellites with an accuracy lower than a centimeter, provided that the satellites are not set at a distance apart greater than about fifty meters. Some fine metrology systems even enable determination of the mutual position of two satellites with an accuracy of the order of one tenth of a millimeter, provided that the satellites are not set at a distance apart greater than one meter.
Instead, coarse metrology systems are characterized by an accuracy not lower than about ten centimeters. However, they are able to operate also when the distance between the satellites is greater than fifty meters, for example also up to distances of twenty kilometers.
By way of example, coarse metrology systems comprise metrology systems based upon the use of the satellite global positioning system (GPS), as well as metrology systems based upon the use of radio-frequency radiation, the latter resorting to considerably complex antenna networks.
As regards, instead, fine metrology systems, known to the art are systems of an at least in part projective type, which envisage that, given two satellites, one of them is equipped with a target formed by a number N of light sources, and the other is equipped with an optical unit, which includes an optoelectronic sensor able to acquire an image of the target, on the basis of which, by means of post-processing, the optical unit itself determines one or more of the aforementioned degrees of freedom.
By way of example, the patent application No. BP1986018 describes a system for determining the position and attitude of a system with six degrees of freedom, and where the number N of light sources of the target is equal to one. However, to enable determination of all six degrees of freedom, the system described in the document No. EP1986018 requires the individual light source to be formed by a coherent-light source such as, for example, a laser, and moreover requires that the optical unit will be able to carry out, in addition to processing of the images of the target, measurements of the power effectively received by the optoelectronic sensor and an angular measurement of rotation of the polarization of the beam of light emitted by the coherent-light source.
In even greater detail, with reference to a first satellite and a second satellite, and assuming that the target is located on the first satellite, the system described in the document No. EP1986018 envisages that the optical unit on board the second satellite will be equipped with three optoelectronic detectors that are able to process images and detect, each, the power associated to the fraction of electromagnetic radiation emitted by the coherent-light source and effectively impinging upon the optoelectronic detector itself. Consequently, the system described in the document No. EP1986018 is not of a purely projective type.
There are on the other hand known also fine metrology systems that do not envisage determination of measurements of power, i.e., metrology systems of a purely projective type. With respect to what is described in the document No. EP1986018, said systems of a projective type require the use of targets formed by a large number of light sources. An example of said metrology systems is provided in the document No. U.S. Pat. No. 7,561,262, where the light sources are formed by reflectors designed to be arranged on the first satellite, which are illuminated by radiation emitted by the second satellite. Moreover known are fine metrology systems of a purely projective type, where the target is formed by a particularly large number of light sources (for example, eight).
The use of targets formed by a large number of light sources involves a greater complexity of construction, and moreover entails an increase in the consumption of electric power both in the case where the light sources are optically active (i.e., they emit light signals) and in the case where they are passive (i.e., they are formed, for example, by reflectors). In fact, in the latter case it is necessary to irradiate the satellite housing the target with a particularly wide electromagnetic-wave front in order to illuminate all the reflectors, with consequent expenditure of electromagnetic power.
Moreover known are so-called “star-tracking” systems. For example, the document No. US2005/213096 describes a system designed to receive light signals coming from a stellar field and to generate a pair of images using an array of mirrors that can be actuated independently of one another. The mirrors are arranged in such a way that, given a stellar target that is to be identified, when this stellar target is framed by the system, the corresponding pair of images collapses into a common focal point, enabling recognition of the stellar target itself. This system hence operates on the hypothesis of light rays coming from infinity, and hence parallel to one another; moreover, it does not enable determination of any quantity regarding the position (distance) of the stars with respect to the system itself.
Finally, systems of the type described in the document No. US2008/111985 are known, where a first image and a second image of a target are formed on two different photosensitive arrays. Quantities of interest are then determined on the basis alternatively of the first image or else of the second image. This system is hence characterized by a certain complexity since it requires the use of two different photosensitive arrays.